1. Field of the Invention
The present invention relates to an electrophotographic imaging apparatus, and more particularly to a nip biasing and backup roll release mechanism in the fuser roll assembly for such an apparatus.
2. Description of the Related Art
In the electrophotographic process commonly used in printers and the like, an electrostatic image is created on photosensitive material such as a belt or roll. Minute electroscopic particles, commonly called toner particles, are applied to the electrostatic image on the photosensitive material. The toner image is then transferred to the desired media, which may include paper, card stock, envelopes, transparencies or the like.
To permanently adhere the toner to the media, the media, with the toner image thereon, is passed through a fuser unit, in which heat and pressure are applied to the image. The heat causes constituents of the toner to flow into the pores or interstices between fibers of the media. Pressure promotes settling of the toner constituents into these voids. As the toner is cooled, it solidifies and adheres the image to the media.
A commonly used fuser is a roll fuser, consisting of two rolls nipped together, with at least one roll being internally heated. The nonheated roll, or backup roll, is urged against the hot roll, to form a fuser nip through which the media passes. Nip pressures in the fuser can be high, often being at least 13 psi.
A common problem with roll fusers is the need to relieve the nip when paper jams occur. It is essential that a user be able to remove simple jams easily, without the need for service calls. Known procedures for clearing jams in a fuser include: allowing the fuser roll to free-wheel in the process direction during jam conditions, by disengaging the fuser drive when the jammed media is pulled; implementing a user activated manual lever to separate the backup roll from the hot roll, and thereby relieve the nip pressure; implementing a backup roll release lever which is activated by opening a jam access door, to thereby relieve nip pressure; or implementing a jam clearance knob which is activated by the user to turn the fuser rolls and expel the jammed media from the fuser without relieving nip pressure.
While the mechanisms described above typically achieve the desired function for clearing media jams, in some situations, and for some machine architectures, each has draw backs and disadvantages. In a machine architecture in which the paper process direction runs perpendicular to the front of the machine, it is often desirable, and frequently necessary, to allow the user to clear jams both from the front of the machine and from the side of the machine. It is further necessary to allow the user to clear jams from both the pre-fuser and the post-fuser areas of the imaging apparatus. Typically, in the aforedescribed machine architecture, the pre-fuser area is accessible from the front of the machine, such that jammed sheets are extricated perpendicular to the process direction. In the post-fuser area, accessed from the side of the machine, the jammed sheets are removed generally parallel to the process direction.
Allowing free-wheeling of the fuser in the process direction is sufficient to clear jammed pieces of media that can be extricated in the same direction as the process direction, such as in the post-fuser area. However, clearing jams from the front of the machine, such as the pre-fuser area, where the jammed media must be removed in a direction that is perpendicular to the process direction, is more difficult. If jammed sheets are removed across the process direction, free-wheeling of the fuser rolls, which does not relieve nip pressure, is not helpful in freeing the jammed media. Pulling on a sheet still nipped between fuser rolls often results in the sheet tearing. When this occurs, it is difficult to remove the torn sheet remnants, which may be small and virtually inaccessible. In extreme cases, removal of the entire fuser may be required to gain access to the remaining torn pieces. This may require a service call by technicians, which can be expensive, and delays having the machine in service.
Incorporating jam clearance knobs may improve the above scenario for clearing jams, since the user will then have means to expel the sheet from the nip, thus reducing the likelihood of tearing. However, the knob will be accessible only from either the front or the back of the machine, unless two knobs are used, which in itself is undesirable in utilizing more space in a relatively compact and crowded housing.
A more desirable jam clearance approach for a machine architecture in which the paper process direction runs perpendicular to the front of the machine is to physically open the fuser roll nip, thereby allowing the media to be extricated from the fuser, with little or no resistance, in either the process direction or the cross process direction. In the past, typical means for opening the nip have included manual levers and levers actuated by opening jam access doors. If a manual lever is used, for the ease and convenience of the user, the manual lever linkage should be accessible from both the front and side locations of the machine. Such an arrangement itself is complicated, utilizing much interior space of the apparatus. In a door actuated lever design, reinforcement of the doors is necessary, to carry the extra load required to force the fuser nip open. Long actuating levers may be required. While either of these approaches may be functional, they are costly and inconvenient solutions.
What is needed is a fuser backup roll release mechanism which can be activated conveniently to clear media jams in the pre-fuser and post-fuser areas as well as at the fuser nip, and which allows removal of the media from the fuser in both the process direction and the cross-process direction.
An additional problem of roll fusers of the type described above is that the typical roll fuser operates at a single nip pressure. This pressure may be greater than the optimal pressure under some circumstances. For example, passing envelopes through printers utilizing roll fusers often results in the envelope becoming wrinkled. If a lower nip pressure were available in the roll fuser, wrinkling would be minimized. Similarly, it may be desirable to utilize higher nip pressures in the roll fuser for card stocks and labels than for envelopes or other standard media. Transparency sheets also may be treated, ideally, with nip pressures different from those used for other media types. Additionally, it may be useful to control fuser roll nip pressure to achieve desired print characteristics, such as glossiness.
What is needed is a roll fuser backup roll biasing mechanism capable of multiple settings, so that preferred nip pressure settings can be implemented for the media being processed.
Another problem encountered with roll fusers is that referred to as compression set of the elastomer covers on the rolls. Compression set, which is a distortion in the shape of the elastomer cover, can occur if the fuser roll and backup roll remain for an extended period of time in a fixed nip relationship under pressure. When the apparatus is used, as the rolls rotate, the nip area transfers about the periphery of the roll as it rotates. If the apparatus sits for an extended period of time without being used, such that the rolls do not rotate, the nip remains fixed in position on each roll. The pressure applied to a discrete area of the roll surface can cause roll cover distortion.
What is needed is a roll fuser backup roll biasing mechanism and control procedure which automatically relieves nip pressure if the apparatus sits for an extended period of time without operating.